1. Field of the Invention
This invention relates to a process for the production of an alkali metal hypophosphite by an ion-exchange membrane electrodialysis method and a process for the separate production of an alkali metal hypophosphite and an alkali metal phosphite by an ion-exchange membrane electrodialysis method.
2. Description of the Prior Art
Alkali metal hypophosphites have been used as synthetic resin stabilizers, catalysts for organic synthesis, reducing agents for industrial use, etc. and in particular, sodium hypophosphite has widely been used as a chemical for electroless plating.
Alkali metal hypophosphites are ordinarily prepared by the reaction of white phosphorus with an aqueous solution of alkali metal hydroxide, during which a side reaction of forming the phosphite unavoidably takes place with formation of the hypophosphite. Thus, there is always a considerable amount of the phosphite in the reaction mixture. The amount of the phosphite, depending on the reaction conditions, reaches 50% when it is formed in the greatest amount. In the general process for the production of an alkali metal hypophosphite, therefore, it is indispensable to provide a step of separating and removing the phosphite from the reaction mixture. Up to the present time, separation of the phosphite has been carried out by precipitating and removing it as calcium phosphite utilizing the fact that calcium hypophosphite is water-soluble, while calcium phosphite is water-hardly soluble. For example, this is accomplished by adding a water-soluble calcium compound as a precipitant to the reaction mixture. When using calcium hydroxide as a precipitant, however, the pH is raised by the precipitating reaction at the initial period, the concentration of calcium ion is lowered and a part of the phosphite remains in the solution so that complete separation of the phosphite is impossible. When using a calcium salt such as calcium chloride, a new impurity such as chloride ion is carried in the reaction system, which must subsequently be removed.
In order to avoid the disadvantage of such a calcium precipitant, it has been proposed in U.S. Pat. No. 2,938,770 to precipitate and separate calcium phosphite with a solution of calcium hypophosphite. In this case, however, it is required to separately prepare calcium hypophosphite as a precipitant by the treatment of sodium hypophosphite with an ion exchange resin and the steps including a regenerating treatment are very complicated.
Another method for separating the byproduced phosphite as a less soluble calcium salt comprises reacting white phosphorus and an alkali metal hydroxide in the presence of calcium hydroxide, for example, by adding white phosphorus to a slurry of calcium hydroxide suspended in an aqueous solution of the alkali metal hydroxide. In this method, the soluble calcium content is contained in the reaction system, so the byproduced phosphite is precipitated as less soluble calcium phosphite, as soon as it is formed, without changing the pH of the solution. Consequently, the phosphite is surely separated and no other impurities are introduced. In the filtrate from which the calcium phosphite has been precipitated and separated, however, there are dissolved excessive calcium hydroxide and soluble calcium hypophosphite in addition to the alkali metal hypophosphite and therefore, it is necessary to remove them.
A method of removing the dissolved calcium content has been proposed in German Patent No. 2,006,632, which comprises first neutralizing the filtrate with carbon dioxide to precipitate the excessive calcium hydroxide in the form of calcium carbonate, further adding sodium carbonate to precipitate calcium carbonate from the calcium hypophosphite and then neutralizing the remaining sodium carbonate with hypophosphorous acid. U.S. Pat. No. 4,379,132 describes a method comprising precipitating and separating a part of the dissolved calcium contents with phosphoric acid or an acid phosphate with the co-production of an insoluble calcium phosphate and converting the calcium hypophosphite dissolved in the filtrate into the sodium salt by the use of an ion exchange resin of sodium type.
Furthermore, U.S. Pat. No. 4,521,391 describes a method comprising adding sodium hydrogen carbonate to a filtrate after separating calcium phosphite to precipitate dissolved calcium hydroxide and further adding sodium carbonate to the filtrate to precipitate and separate the dissolved calcium hypophosphite as calcium carbonate.
As described above, the method of the prior art in which the phosphite is subjected to solid-liquid separation as the less soluble calcium salt must necessarily be carried out in batchwise manner and thus needs steps of separating calcium phosphite and removing soluble impurities resulting from the precipitants, so that the process is complicated and is of low efficiency.
Sodium hypophosphite is very useful as a reducing agent for electroless plating, but of late, requirements for the quality of sodium hypophosphite have become severer since the application of the electroless plating process has spread in the high technical field and it has eagerly been desired to provide high purity sodium hypophosphite with a decreased content of impurities having bad influence upon the adhesiveness of plating coatings and the reactivity of plating solutions. However, the prior art cannot fully deal with these problems.